In recent years, a display device using a self-light emitting element typified by an electroluminescence (EL) element and the like has been researched and developed to replace a liquid crystal display (LCD) using a liquid crystal element for a pixel. The display device using a self-light emitting element has the advantages of high image quality, wide viewing angle, and being thin and light because of requiring no backlight. Therefore, it is expected to be widely used as a display panel of a mobile phone or as a display device.
On the other hand, the portable information equipment is required to have high added value as the intended purpose thereof is diversified. Thus, the equipment provided with a sub-display screen on the back side of the main display screen has been provided recently.
Furthermore, it is possible to browse the Web pages on the portable information equipment and the application is improved. Therefore, the equipment whose screen can be switched between the horizontal direction and the vertical direction has been proposed.
In the portable information equipment provided with the sub-display screen besides the main display screen, not only the volume occupied by a module which includes a backlight or the like but also the volume occupied by a substrate which mounts a control IC or the like for driving the module is to be paid attention to. Particularly, in the recent portable information equipment, reduction in weight, thickness, and size is considerably advanced and trades off between a high added value. For example, when the portable information equipment which can display on both screens is fabricated by using a liquid crystal display, a display portion thereof is quite difficult to be thin since a backlight or the like has to be disposed between the two screens.
In addition, the number of pixels of a display device is generally different in the vertical and the horizontal directions. Accordingly, when switching between vertical and horizontal display, it is necessary for example that image signals of one frame are temporarily stored in a frame memory provided additionally, and then the format of the image signals are converted in accordance with the number of pixels in the vertical and the horizontal directions.
In accordance with the foregoing, the invention provides a display device which is thin and capable of displaying on both screens, and which can be made into a small module.